Electrochemistry: Electrolysis and Fuel Cells
Split Stuff With Electricity ⚡
Introduction
1. Introduction
Okay, electrochemistry. The name sounds heavy but the idea is genuinely cool: you run electricity through a melted or dissolved compound and it rips apart into its elements. That's electrolysis. Flip the whole thing around and you get a fuel cell that burns hydrogen quietly to make electricity, with only water coming out.
Here's the cheat code for the whole chapter: ionic compounds are made of ions, and ions only move when the compound is melted or dissolved. Once they can move, the power supply drags the positives one way and the negatives the other, and they turn back into elements at the electrodes. Lock that picture in and every product prediction is just two short rules. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact move the marker wants. Let's go ⚡
Here's the cheat code for the whole chapter: ionic compounds are made of ions, and ions only move when the compound is melted or dissolved. Once they can move, the power supply drags the positives one way and the negatives the other, and they turn back into elements at the electrodes. Lock that picture in and every product prediction is just two short rules. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact move the marker wants. Let's go ⚡
2. What Electrolysis Is
Electrolysis is breaking down an ionic compound, when it's melted or dissolved in water, by passing an electric current through it. The "melted or dissolved" bit is the whole secret: in a solid the ions are locked in place and can't go anywhere, but melting or dissolving sets them free to move, and moving ions are what carry the current. The liquid that gets broken down is the electrolyte.
Key idea🔑 Key idea: Electrolysis = break down an ionic compound (molten or dissolved) with an electric current. The ions must be free to move, so solids don't work. ⚡
Worked example
Worked Example: Defining Electrolysis
Worked Example: Say It in Four Chunks 🧩
What is meant by the term electrolysis?
- 1Break the definition into chunks: break down · ionic compound · molten or dissolved · electric current.
- 2Put it together: electrolysis is breaking down an ionic compound, when molten or in solution, by passing an electric current through it. Not joining molecules, not burning a fuel.
So that's the definition, four chunks, all present. ✅
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3. The Parts of the Cell
Every electrolysis cell has three parts to name. The electrolyte is the melted/dissolved compound. The cathode is the electrode on the negative terminal. The anode is the electrode on the positive terminal. The electrodes are usually inert (platinum or carbon/graphite), meaning they conduct the current but don't react with anything.
Key idea🔑 Key idea: Remember PANC: Positive Anode, Negative Cathode. Inert electrodes (platinum, graphite) conduct but don't react. 🔋
Worked example
Worked Example: Labelling a Cell
Worked Example: PANC in Action 🏷️
X is on the positive terminal, Y is on the negative terminal, Z is the liquid between them. Label X, Y and Z.
- 1Positive → anode (X). Negative → cathode (Y).
- 2The liquid that carries the current is the electrolyte; it has to be melted or dissolved (aqueous) to conduct.
So X = anode, Y = cathode, Z = aqueous salt. ✅
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4. Where the Products Form
Every product in this chapter lands in one of two places. Metals, or hydrogen, form at the cathode (negative). Non-metals other than hydrogen form at the anode (positive). Why? In an ionic compound the metal is a positive ion and the non-metal a negative ion, and opposite charges attract. Hydrogen is the exception: it's a non-metal, but in acids and water it's there as positive ions, so it forms at the cathode like a metal. At Core you only say which element forms where; you don't describe how electrons move.
Key idea🔑 Key idea: Cathode (−) → metals or hydrogen. Anode (+) → non-metals other than hydrogen. Positive ions go to the negative electrode. 🔀
Worked example
Worked Example: What Can Form at the Cathode?
Worked Example: Spot the Odd One Out 💧
Which list contains only substances that can form at the cathode: (A) chlorine, copper, hydrogen; (B) copper, hydrogen, zinc; (C) hydrogen, oxygen, sodium; (D) bromine, lead, oxygen?
- 1The cathode gives metals or hydrogen, so one non-metal other than hydrogen rules a list out: (A) has chlorine, (C) has oxygen, (D) has bromine and oxygen.
- 2Only (B) is all metals plus hydrogen.
So the answer is B. ✅
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5. Melted Compounds
When a binary compound (a metal + a non-metal) is melted, there's no water to complicate things, so the rule is dead simple: the metal forms at the cathode (negative) and the non-metal forms at the anode (positive). The classic example is lead(II) bromide: a silvery bead of lead at the cathode and a brown bromine vapour at the anode.
Key idea🔑 Key idea: Melted compound → metal at the cathode (−), non-metal at the anode (+). Match the product to the sign, not the first letter. 🧊
Worked example
Worked Example: Molten Lead(II) Bromide
Worked Example: Read the Sign 🟤
Molten lead(II) bromide is electrolysed. Electrode P is positive, electrode Q is negative. What forms at each?
- 1Q is negative (cathode) → the metal, lead (a silvery bead).
- 2P is positive (anode) → the non-metal, bromine (a brown vapour).
So bromine at P, lead at Q. ✅
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6. Predicting a Melted Salt
Once you've got lead(II) bromide, every other melted metal salt is the exact same template, so you never have to learn a new one. Zinc chloride, potassium iodide, calcium chloride, sodium chloride: in every case the metal lands at the cathode and the non-metal comes off at the anode. Just identify the metal and the non-metal in the formula and place them.
Key idea🔑 Key idea: Every melted metal salt follows one template: metal → cathode (−), non-metal → anode (+). 🔮
Worked example
Worked Example: Molten Zinc Chloride
Worked Example: Place the Two Elements 🧲
Molten zinc chloride is electrolysed. What forms at the positive and negative electrodes?
- 1Find the two elements: zinc (metal) and chlorine (non-metal).
- 2Metal → negative electrode; non-metal → positive electrode.
So chlorine at the positive electrode, zinc at the negative electrode. ✅
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7. Concentrated Sodium Chloride
Dissolve a compound in water and water is there too, so there's more than one possible product. At Core you learn the results for two named solutions as facts. For concentrated sodium chloride: colourless hydrogen at the cathode (sodium is much more reactive than hydrogen, so it stays in solution as ions) and pale yellow-green chlorine at the anode. The chlorine test: it bleaches damp litmus paper (blue litmus may turn red for a moment first). You don't predict products for other solutions at Core.
Key idea🔑 Key idea: Concentrated sodium chloride → hydrogen at the cathode, chlorine at the anode. Learn it as a fact. Chlorine bleaches damp litmus paper. 🧂
Worked example
Worked Example: The Chlorine Test
Worked Example: Red Then White 🩸
Concentrated sodium chloride is electrolysed. The anode gas is put on damp blue litmus paper. What's seen, and which gas?
- 1Concentrated chloride at the anode → chlorine.
- 2Chlorine is acidic and a bleach, so the blue litmus turns red, then is bleached white.
So: turns red then bleached white, caused by chlorine. ✅
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8. Dilute Sulfuric Acid
Dilute sulfuric acid is basically acidified water, so electrolysing it just splits water. You get hydrogen at the cathode and oxygen at the anode. Because water is H₂O (two hydrogens to one oxygen), the hydrogen comes off at twice the volume of the oxygen. Both gases are colourless.
Key idea🔑 Key idea: Dilute sulfuric acid → hydrogen (cathode) and oxygen (anode), in a 2 : 1 volume ratio. The bigger volume is the hydrogen. 💦
Worked example
Worked Example: Hydrogen and Oxygen Volumes
Worked Example: Double the Hydrogen 📏
20 cm³ of gas collects at the cathode. What volume collects at the anode, and which gas?
- 1Cathode → hydrogen. Anode → oxygen. There's twice as much hydrogen as oxygen.
- 2So oxygen is half of 20 cm³.
So 10 cm³ of oxygen. ✅
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Answer
9. Electroplating
Electroplating uses electrolysis to coat an object in a thin layer of metal, to make it look good and to protect it from corrosion. The setup has a fixed recipe: the object is the cathode, the anode is the pure plating metal (it dissolves to keep the solution topped up), and the electrolyte is a salt of the plating metal (like silver nitrate for silver).
Key idea🔑 Key idea: Electroplating: object = cathode, plating metal = anode, electrolyte = a salt of that metal. Done for appearance and corrosion resistance. ✨
Worked example
Worked Example: Why We Electroplate
Worked Example: Two Real Reasons 🎯
Four statements: (1) improves appearance; (2) insulates the object; (3) resists corrosion; (4) makes it softer. Which explain why we electroplate?
- 1Plating gives a shiny surface → (1) correct. Plating protects from corrosion → (3) correct.
- 2It doesn't insulate (metal still conducts) and doesn't soften it.
So the answer is 1 and 3. ✅
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10. Hydrogen–Oxygen Fuel Cells
A hydrogen–oxygen fuel cell runs the whole thing backwards: instead of using electricity to split water, it uses hydrogen and oxygen to produce electricity, and water is the only chemical product. Hydrogen and oxygen go in, water comes out, and the cell makes electrical energy only while both gases are supplied. At Core you don't need to compare it with a petrol engine: just learn what goes in, what comes out, and that it makes electricity.
Key idea🔑 Key idea: Fuel cell: . In: hydrogen and oxygen. Out: water, the only chemical product. Makes: electricity. 🚀
Worked example
Worked Example: The Fuel Cell Equation
Worked Example: Water on the Right 💧
Which equation is the overall reaction in a hydrogen–oxygen fuel cell?
- 1Hydrogen joins oxygen to make water, so water is on the right.
- 2Watch out: the reversed version (water on the left) is electrolysis, not the fuel cell.
So the answer is . ✅
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